1. Field of Invention
Design and construction of road and street asphalt pavements with concrete subbase.
2. Description of Prior Arts
Asphalt pavements constitute near 90% of US road construction. It can be explained by the comparatively moderate initial cost of these pavements; it is less than that for concrete pavements by 20% on average. However maintenance cost of asphalt pavements is considerably higher than that for concrete pavements. Cracking and consequent failure of asphalt surface course of asphalt pavement are caused usually by considerable deformations of crushed stone or other granular materials subbase and base course of this pavement. Moreover, damages of asphalt pavement can be caused by the fatigue stresses of surface course due to deformations of lower courses of this pavement.
Deformations of flexible lower courses of asphalt pavement should be limited to reduce cracking of the surface course of this pavement. It can be achieved by the use of concrete subbase or at least cement treated subbase instead of crushed stone the base course of flexible pavement should remain flexible. The increase of rigidity of asphalt pavement should reduce the cracking of surface course with the corresponding reduction of the maintenance cost of this pavement. It is important to reach reduction of the maintenance cost of this pavement without consideration increase of its initial cost due to use of concrete subbase instead of the crushed stone one.
The cost of concrete subbase s determined by the value of flexural strength of concrete, which is estimated by the value of modulus of rupture. The choice of modulus of rupture of concrete of subbase is determined merely by the economical reasons. Asphalt pavement is a composite structure, and the increase of capacity of subbase means the corresponding increase of capacity of pavement as a whole.
As applied to composite concrete pavement the increase of flexural strength of lean concrete subbase or lower layer means the possibility of the corresponding reduction of normal concrete surface course of this pavement. Design procedure of the normal concrete pavement results in the certain value of normal concrete thickness. The sense of composite concrete pavement of the identical capacity is in the reduction of consumption of normal concrete with high cost crushed granite as a coarse aggregate by replacing of a part of this concrete by subbase or lover layer cheaper concrete.
The increase of capacity of pavement depending on the increase of flexural strength of subbase can be estimated by the example of composite concrete pavement designed according to Portland Cement Association Engineering Bulletin (Thickness Design for Concrete Highway and Street Pavements, Portland Cement Association, EB 109P). This design procedure indicates a thickness for two-layer concrete pavement equivalent to a given thickness of normal concrete.
Increase of equivalent normal concrete thickness of composite pavement due to increase of flexural strength of concrete subbase can be considered approximately as a measure of possible reduction of thickness of normal concrete surface course of this pavement. The design chart for composite concrete pavement with lean concrete subbase of modulus of rupture in the range from 150 to 450 psi is presented on the FIG. B1, Appendix 2 of said Portland Cement Association Engineering Bulletin. It allows estimation of equivalent normal concrete thickness of composite concrete pavement corresponding to the different combinations of thickness of lean concrete subbase and normal concrete surface course of pavement.
The estimations of equivalent normal concrete thickness of pavement corresponding to the lean concrete 4-inch thickness subbase of modulus of rupture in the range from 150 to 450 psi and different values of thickness of surface course were determined according to this design chart. It allows estimation of the change of equivalent thickness of composite pavement depending on the change of lean concrete flexural strength of subbase. Moreover, relative increase of this thickness depending on the increase of modulus of rupture of lean concrete of subbase were carried out. The equivalent normal concrete thickness corresponding to the value of modulus of rupture equal to 150 psi is considered as 1,0. Results of these calculations are presented in Table 1.
As can be seen from the Table 1, equivalent normal concrete thickness of composite pavement is increased at least 15% due to the increase of modulus of rupture of concrete of subbase from 150 to 450 psi. It can be considered as estimation of corresponding reduction of the thickness of normal concrete surface course.
TABLE 1Modulus of rupture of normal concrete ofModulus of rupture of normal concrete ofsurface course in the range 600 to 700 psisurface course in the range 500 to 600 psiModulus of rupture of 4-inch thicknessModulus of rupture of 4-inch thickness leanlean concrete subbase, psiconcrete subbase, psiThickness150250350450150250350450of normalEquivalent normal concrete thickness of composite pavement (inch) and relativeconcreteincrease of this thickness depending on the increase of modulus of rupture of leansurfaceconcrete of subbase (equivalent normal concrete thickness corresponding to the valuecourse (inch)of modulus of rupture equal to 150 psi is considered as 1.0)7 8.5/1.0 9.1/1.07 9.5/1.12 9.9/1.2 8.6/1.0 9.2/1.0710.0/1.1610.4/1.2 8 9.6/1.010.2/1.0710.7/1.1111.2/1.16 9.8/1.010.5/1.0711.0/1.1211.5/1.17910.6/1.011.3/1.0711.8/1.1112.3/1.1610.9/1.011.5/1.0612.2/1.1212.6/1.1610 11.6/1.012.4/1.0712.9/1.1113.4/1.1611.9/1.012.6/1.0613.4/1.1213.8/1.16
The same results were obtained by estimation of the ratio of equivalent normal concrete thickness of composite pavement to physical one depending on the change of flexural strength of lean concrete of subbase. Average estimations of this ratio corresponding to the values of modulus of rupture of concrete in the range from 150 to 450 psi are presented in the Table 2.
TABLE 2Modulus of rupture ofModulus of rupture ofnormal concrete of surfacenormal concrete of surfacecourse in the rangecourse in the range600 to 700 psi500 to 600 psiModulus of rupture of leanModulus of rupture of leanconcrete of subbase, psiconcrete of subbase, psiThickness150250350450150250350450of leanRatio between equivalent normal concreteconcretethickness of composite concrete pavementsubbase in.and physical one of this pavement40.8030.8560.9020.9360.8290.8770.9230.96250.7860.8350.8970.9120.8100.8580.9060.95060.7070.8190.8640.8640.7930.8400.8950.944
It is evident that the increase of flexural strength of concrete subbase is efficient as applied to composite concrete pavements. It can be efficient for asphalt pavements as composite structures.
The compressive and flexural strengths of lean concrete are determined to a great extent by the quality of coarse aggregate. Lean concrete can be produced when local or recycled, relatively cheap coarse aggregates are available; the cost of concrete is determined to a large degree by the cost of coarse aggregate. The use of cheap small grains coarse aggregates is the one of the ways of obtaining of lean and not only lean concrete.
Small grains crushed limestone is one of the cheapest aggregates. According to the US Geological Survey, crushed limestone constitutes 71% of total weight of coarse aggregates for concrete produced in USA. This product of grading finer than 9.5 mm usually is not used as a coarse aggregate. Utilization of great deposits of crushed limestone finer than 9.5 mm and especially finer than 4.75 mm (from 10 to 25% of the total volume of quarrying) are urgent for aggregate industry. The object of design of composite concrete pavements is to obtain the highest concrete strength of subbase and lower layer of this pavement with the cheapest coarse aggregate and with the moderate consumption of cement.
Crushed limestone of regular sizes is most popular coarse aggregate concrete in the US building practice. The choice of coarse aggregate cheaper than crushed limestone of regular sizes, which does not require additional consumption of cement, is the way to reduce cost of concrete subbase of asphalt pavement and this pavement as a whole.